Inflammatory Breast Cancer (IBC) is one of the most lethal forms of breast cancer, where patients show less than a five percent survival rate beyond five years when treated with surgery or radiation therapy. IBC cells secrete differentiation factors, stimulate vasculogenesis, and invade as tumor emboli located within a de novo formed vessel. In IBC, the embolus maintains cell-cell attachments as it moves through the vessel and lodges within a dermal lymph node, causing the IBC inflammatory phenotype. Although the molecular changes that dictate IBC invasion remain to be fully elucidated, IBC cell lines and human tissues, overexpress E-cadherin, Epidermal Growth Factor Receptor (EGFR), and Human Epidermal Growth Factor 2 (HER2) which are cell surface proteins associated with maintaining tumor spheroid integrity, increase IBC tumor growth rate, and invasion and metastasis. Since IBC tumor cell emboli are more efficient at forming metastases and are more resistant to chemo and radiotherapy than single cells, it seems feasible to prevent IBC with a compound that has the ability to disintegrate the cell spheroids. We recently published that Ganoderma lucidum (Reishi), a mushroom used in Traditional Chinese Medicine, selectively inhibits IBC cell viability, but not viability of non- cancerous mammary epithelial cells. Also, we have shown that Reishi inhibits the expression of key proteins important in IBC progression, and tumor spheroid fomation. The main hypothesis of the present proposal is that Reishi inhibits cell surface protein expression, which contributes to disintegration of the tumor spheroids that are created during IBC progression, resulting in invasion inhibition combined with altered intracellular signaling. In support of this hypothesis, preliminary studies in our laboratory showe that Reishi 1) downregulates a number of cancer promoting molecules, 2) Reishi inhibits EGFR and E-cadherin protein abundance in vitro and in vivo. 3) Reishi inhibits IBC tumor growth and tumor weight in severe combined immunodeficient (SCID) mice by 58% and 45%, respectively. We propose that Reishi downregulates the expression of cell surface proteins involved in the progression of IBC and chemosensitize the IBC cells to EGFR/HER2 therapy treatment. Therefore, the main goal of this proposal is to validate the inhibitory role of Reishi on IBC progression via the inhibition of plasma membrane (PM) proteins and subsequent EGFR intracellular signaling events. To test our hypothesis, we propose 1) to isolate and identify PM proteins in non- cancerous mammary epithelial cells and in IBC cells treated with Reishi and 2) to investigate the therapeutic potential of Reishi in IBC cells, focusing on EGFR/HER2 signaling cascades. The proposed dissection of the molecular mechanisms of IBC is expected to significantly advance the current understanding and development of targeted therapeutic agents for IBC as well as contribute to a rapid and accurate diagnosis of this mortal disease.